Ultrasound image enhancement and super-resolution

ABSTRACT

Techniques to improve resolution in an ultrasound system are disclosed. An exemplary apparatus is a portable ultrasound probe having transducer elements and supporting electronics within the probe. The beam is shaped to split the resolution to sub-pixel accuracy. Super resolution sample technique based on interpolation can be used to further increase resolution. In one embodiment the ultrasound system supports ½ crystal physical resolution and ¼ crystal digital resolution.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. application Ser. No.14/292,431, filed on May 30, 2014, which claims the benefit of U.S.Provisional Application No. 61/829,894, filed on May 31, 2013, thecontents of both are hereby incorporated by reference.

This application incorporates by reference commonly owned U.S. patentapplication Ser. No. 14/214,370, entitled “Ultrasound Probe”, filed onMar. 14, 2014.

FIELD OF THE INVENTION

The present invention is generally related to improving image quality inultrasound imaging systems. More particularly, the present invention isdirected to achieving high image quality in an ultrasound system have areduced number of transducer elements using super-sampling andintelligent scan line conversion.

BACKGROUND OF THE INVENTION

Ultrasound imaging machines include a transducer probe that includespiezoelectric crystals to generate sound waves and also detect thereflected waves. In a conventional ultrasound machine the transducerprobe is connected to an external processing box by a cable, where theexternal processing box has electronics to generate high frequencyvoltage pulses sent to the transducer probe, receive detected signalsfrom the transducer probe, and perform signal processing and scan lineconversion to reconstruct the image.

Referring to FIG. 1, in a conventional ultrasound imaging machine thecable is typically several meters long (e.g., 2 m) and contains 48 to256 micro-coaxial cables, where the number of micro-coaxial cablesscales with the number of transducer elements in the transducer probe.The micro-coaxial cables are expensive and have other disadvantages. Inparticular, the micro-coaxial cables introduce a cable loss and a cableimpedance. For example, a conventional 2 m cable might have acapacitance of 203 pF, while a transducer element could have acapacitance on the order of 5 pF. Additionally, a 2 m cable mayintroduce a 2 dB attenuation. The cable introduces a large capacitiveloading, which makes it impractical to perform fine grained apodizationof the transmitted voltage pulses sent to the transducer probe.

In ultrasound systems the spatial resolution of the image is determinedby the size of the piezoelectric crystal (“crystal”) elements of thetransducers and the number of such crystal elements. Higher resolutiontypically implies smaller crystals and larger crystal arrays. Largercrystal arrays lead to more expensive systems and limitations on thephysical layout and cabling of the system. In particular, the number ofmicro-coaxial cables required increases with the number of crystalelements. Thus, in the prior art increasing spatial resolution requiresmore crystal elements and more complex and costly cables.

Thus, ultrasound imaging systems are more expensive than desired. Thisis due, in part, to need for a large number of transducer crystals andthe cost and complexity of the micro-coaxial cables and associatedelectronics. For example, in 2014 a commercial ultrasound imaging systemmay cost $30-50 k. Additionally, another problem in the prior art isthat quality of the scan line processing to reconstruct the ultrasoundimage is poorer than desired. In particular, the scan line conversion atthe distal end of the ultrasound beam results in poor resolution due toa lack of signal strength, loss of beam focus, and inadequate spatialbinning.

The present invention was developed to address the above describedproblems in the prior art.

SUMMARY OF THE INVENTION

An apparatus, system, method, and non-transitory computer readablemedium to provide super-resolution in an ultrasound image scanner isdisclosed. The firing sequence of the transducer elements may beselected to achieve sub-pixel resolution, up to half crystal physicalresolution. Additionally, interpolation may be used to achieve ¼ crystaldigital resolution. The transducer elements, beam forming elements, scanconversion, and analog front end may be included within a handheldprobe. The probe may also include auto-calibration and features tosupport fine temporal and spatial apodization. In a sector scanningembodiment, a zone-based interpolation technique may be used toselectively increase interpolation in regions with poor spatial binning.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art ultrasound imaging system;

FIG. 2 illustrates an exemplary ultrasound probe in accordance with anembodiment of the present invention;

FIG. 3 illustrates a beam firing sequence in accordance with anembodiment of the present invention;

FIGS. 4A and 4B illustrates aspects of fine-grained temporal and spatialapodization in accordance with an embodiment of the present invention;and

FIG. 5 illustrates scan line interpolation in accordance with anembodiment of the present invention.

FIG. 6 illustrates scan line interpolation in a radial zone scheme inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 2 illustrates an exemplary system in accordance with an embodimentof the present invention. A handheld probe has a housing 201 thatcontains a transducer array 205 having individual transducer elements207, analog front end (AFE) 210, a beamformer 220, signal processing225, scan conversion 230, ultrasound engine 235, and other probeelectronics 240. The transducer array 205 includes an array oftransducer elements 207, such as an array of piezoelectric crystals, togenerate an ultrasound beam in a transmit mode and to detect reflectedultrasound signals in a receive mode.

In one embodiment the transducer elements are disposed in a detachabletransducer head 270 that permits different transducer heads to beattached. This permits, for example, different geometric arrangements ofthe transducer elements and/or replacements for defective heads. Acalibration module 260 provides auto-calibration of the probe in situand supports replacing/substituting different transducer heads. Theself-calibration of the array also allows superior control over thescanning process. In one embodiment the handheld probe supports lineararray and sector scanning among other options.

The handheld probe also includes the analog and digital elements togenerate the high voltage pulses for the transducer elements to generateultrasound pulses. The digital electronics and analog front end 210include voltage high voltage pulsers and delay elements permit the gainand delay of each high voltage pulser of the crystal segment to beprecisely controlled. Additionally, the handheld probe includes signalprocessing electronics 225 to process the returned pulses andreconstruct the ultrasound image. The electronics and signal processingof the handheld probe also includes at least one processor (not shown)and associated memory.

The handheld probe supports scan conversion and outputs ultrasoundimages through either a wireless or wired connections, such as a USBport 245 or a wireless LAN connection 250. As all of the criticalelectronics are located within the probe there is no capacitive loadingissues as in conventional ultrasound systems that use micro-coaxialcables to couple signals to and from the probe to an external processingbox.

An individual transducer element 207 may be implemented as apiezoelectric crystal. In one embodiment the transducer array 205 has ashort segment of crystals, such as 64 to 128, although more generallyother numbers of crystal elements may be used. That is, the number ofpiezoelectric crystals is selected to be significantly less than the 128to 256 elements in many conventional systems in order to reduce thesize, cost, and complexity of the probe.

The handheld problem includes a module 222 to support super-resolutionbeam shaping and firing sequences. Module 222 may comprise hardware andsoftware, such as digital waveform generators. In one embodiment asuper-resolution mode in the spatial dimension is supported by selectingthe gain and delay of pulses to the transducer array to achievesub-crystal (i.e., sub-pixel resolution). Additional interpolation 232may be provided in the software domain on a processor to furtherincrease resolution when scan line conversion is preformed. Thissuper-resolution thus provides at least a factor of two-to-four moresamples, thus permitting a short segment of crystals (e.g., 64 to 128)to be used to obtain high resolution images.

FIG. 3 illustrates aspects of a firing sequence for achieving sub-pixelresolution in accordance with an embodiment of the present invention. Inone embodiment a select number of piezoelectric crystals are fired at agiven time and translated in a sequence to move the transmitted beamfocal point by ½ crystal (i.e., ½ pixel). As a result, the center ofdistribution of the pulse is shifted ½ crystal at a time in each firingsequence at T0, T1 . . . TN. This can include 1) varying the number ofcrystals driven in a particular firing cycle and 2) shifting theposition of crystals activated. For example, FIG. 3 illustrates alteringthe firing cycle to have either 7 or 8 piezoelectric crystals active andalso translating the distribution of active crystals that are fired. Forexample, at time T0, crystal elements C1 to C7 are fired, having aspatial distribution center at the middle of crystal C4. At time T1,crystals C1 to C8 are fired, shifting the center to the beginning ofcrystal C5. At time T2, crystals C2 to C9 are fired, shifting the centerto the middle of crystal C5. As a result, in each transmit firing cyclethe center of the pulse is shifted by ½ crystal resolution.

Additional interpolation may be used to achieve ¼ digital resolution(super-sample resolution), as indicated by the dashed lines. A varietyof different interpolation techniques may be used to perform the digitalinterpolation, such as linear interpolation or other types ofinterpolation, such as higher order orthonormal interpolation or Fourierinterpolation.

Referring to FIGS. 4A and 4B, in one embodiment the beam shaping hasfine gained control of gain and beam shape. In one embodiment there isfine-grained spatial and temporal apodization may be performed. FIG. 4Aillustrates that the gain may be controlled for each piezoelectricelement in a firing sequence according to a gain profile 405. FIG. 4Billustrates that digital waveform generation techniques 410 may be usedto approximate an ideal pulse waveform. That is, in one embodiment theapodization is fine grained in that there is that the control of thegain, bandwidth, and delay is less than 1%. This may include, forexample, providing digital waveform generators to provides precisecontrol of the transmit pulse waveforms for each individual crystal inthe firing sequence and the use of highly accurate digital to analogconverters (DACs), such as 14 bit or better DACs. In one embodiment thegain and delay of each HV pulser of the crystal segment is adjusted toobtain an effective beam shape that splits the resolution to a sub-pixel(sub-crystal) accuracy. This kind of beam forming also compensates forand improves beam focus on the transmission side. Similar time varyingfocus coefficients are employed on the receiving side to improve spatialfocus.

FIG. 5 illustrates the use of interpolation to improve resolution bygenerating additional interpolated samples for the scan line processingsued to reconstruct the ultrasound imaging and obtain better spatialaccuracy. FIG. 5 shows a sequence of scan lines in an image grid.Samples may be taken on scan lines and also on interpolated lines.Super-resolution sampling methods can be used to interpolate along thetangential direction (with respect to a scan line), to obtain betterspatial accuracy. As illustrative examples, super-resolution samplingmethods can include orthonormal expansion methods such as Legendresampling, Tchebyshev sampling, Sinc sampling, or multirateinterpolation.

Embodiments of the present invention may be applied to sector scanning.In one embodiment the amount of super-resolution is varied radially fromthe transducer, with more super-sampling at the distal end and verylittle at the transducer end. That is, in one embodiment theinterpolation is performed in regions which traditionally have a lack ofsignal strength, loss of beam focus and inadequate spatial binning, asindicated in bin region 505, which illustrates the use of interpolation(dashed lines) to increase the number of samples, via interpolation fromneighboring pixels. In regions distal to the transducer end thetransmitted ultrasound beam may be spread out and the reflected beamhighly attenuated. This improves scan-line conversion at the distal endof the beam which ordinarily suffers from problems such as lack ofsignal strength, loss of beam focus and inadequate spatial binning.

FIG. 6 illustrates a radial zone approach for performing interpolationin sector scanning. Three zones (Z1, Z2, Z3) are shown to illustrateprinciples of operation, although it will be understood that anarbitrary number of zones may be utilized. In a zone proximate thetransducer end, such as zone 1, there is a high density of samples alongthe scan lines such that an individual pixel bin would have a number ofsamples. However, in radial zones further away from the transducer end,the density of real samples decreases and there is inadequate spatialbinning of samples along the scan lines. Thus interpolation may beutilized selectively in zones farther away from the transducer end. Forexample, in zone 2, the samples along the scan line (solid dots) areillustrated along with interpolated sample (crosses) along radial liner1. In zones with even lower density of samples, such as zone 3, moreaggressive interpolation may be used, in this case there is a furtherdoubling of the amount of interpolation performed along radial line r2.

In one embodiment a user can select the type of binning utilized. Thatis, an individual pixel bin includes a number of samples along scanlines and interpolated samples. A gray scale (or color for color images)must be determined for the pixel. In one embodiment the options includean average, a median, a max, a min, a root mean square, or an arithmeticmean. This selection provides additional control for the clinician toadjust performance based on individual preferences. For example,selecting a “max” would ordinarily generate a more speckled image thanselecting an “average.”

While an exemplary apparatus has been described, additional details onan implementation of a portable ultrasonic probe is described incommonly owned U.S. patent application Ser. No. 14/214,370 “UltrasoundProbe”, filed on Mar. 14, 2014, which is incorporated by reference.

While the invention has been described in conjunction with specificembodiments, it will be understood that it is not intended to limit theinvention to the described embodiments. On the contrary, it is intendedto cover alternatives, modifications, and equivalents as may be includedwithin the spirit and scope of the invention as defined by the appendedclaims. The present invention may be practiced without some or all ofthese specific details. In addition, well known features may not havebeen described in detail to avoid unnecessarily obscuring the invention.In accordance with the present invention, the components, process steps,and/or data structures may be implemented using various types ofoperating systems, programming languages, computing platforms, computerprograms, and/or general purpose machines. In addition, those ofordinary skill in the art will recognize that devices of a less generalpurpose nature, such as hardwired devices, field programmable gatearrays (FPGAs), application specific integrated circuits (ASICs), or thelike, may also be used without departing from the scope and spirit ofthe inventive concepts disclosed herein. It will be understood thatembodiments of methods of the present invention may also be tangiblyembodied as a set of computer instructions stored on a computer readablemedium, such as a memory device.

What is claimed is:
 1. A method of providing super-resolution in anultrasound imaging system having a handheld probe includingpiezoelectric transducer having an array of piezoelectric crystals,comprising: generating ultrasound pulses, including performing beamforming to generate voltage pulses within the handheld probe in a firingsequence in which a selection of active piezoelectric crystals is variedover time in a sequence selected to shift a spatial center of atransmitted ultrasound pulse over the firing sequence to achieve atleast ½ piezoelectric transducer crystal physical resolution; detectingreflected ultrasound pulses within the handheld probe; performing scanconversion within the handheld probe; and outputting ultrasound imagedata via a digital interface.
 2. The method of claim 1, wherein thefiring sequence is selected to achieve ½ piezoelectric transducercrystal physical resolution and interpolation of detected reflectedultrasound pulses is performed to achieve ¼ piezoelectric crystaldigital resolution.
 3. The method of claim 1, wherein the selection ofactive piezoelectric crystals is varied by performing at least one ofvarying the number of piezoelectric crystals driven and shifting aposition of piezoelectric crystals activated.
 4. The method of claim 3,wherein the selection comprises sequentially varying a spatialdistribution of active piezoelectric crystals to shift a transmittedbeam focus by ½ piezoelectric transducer crystal physical resolutionover the firing sequence.
 5. The method of claim 4, further comprisingperforming interpolation in the scan conversion to increase a number ofsamples in scan line conversion.
 6. The method of claim 1, wherein theselecting comprises varying a number of active piezoelectric crystalsover the firing sequence.
 7. The method of claim 1, wherein theselecting comprises varying a spatial distribution of activepiezoelectric crystals over the firing sequence.
 8. The method of claim1, further comprising detecting reflected ultrasound pulses in theultrasound probe with a sub-pixel resolution.
 9. The method of claim 8,applying time varying focus coefficients for the detected ultrasoundpulses to improve spatial focus.
 10. The method of claim 1, whereinperforming scan conversion includes performing scan line conversion ofdata received from reflected ultrasound pulses, including performing, ina scan line domain, interpolation of samples along a tangentialdirection.
 11. The method of claim 10, wherein the interpolationcomprises at least one of wavelet interpolation and splineinterpolation.
 12. The method of claim 10, wherein the interpolationcomprises an orthonormal expansion sampling method.
 13. The method ofclaim 12, wherein the orthonormal expansion is selected from the groupconsisting of Legendre sampling, Tchebyshev sampling, and Sinc sampling.14. The method of claim 10, further comprising varying interpolation inradial zones to increase a number of samples in a distal zone regionwith respect to the transducers.
 15. An ultrasound probe, comprising: aprobe housing; an array of piezoelectric transducers disposed in atransducer head detachable attached to the probe housing, wherein eachpiezoelectric transducer in the array includes a piezoelectric crystal;a beamformer disposed in the probe housing, the beamformer includingbeam forming and control electronics to shape a gain and a delay ofvoltage pulses coupled to the array of the piezoelectric transducerswith a firing sequence in which a selection of active piezoelectriccrystals is varied over time in a sequence selected to shift a spatialcenter of a transmitted ultrasound pulse over the firing sequence toachieve at least ½ piezoelectric transducer crystal physical resolution;and scan conversion electronics in the probe housing to convert detectedreflected ultrasound pulses into ultrasound images.
 16. The ultrasoundprobe of claim 15, wherein the firing sequence is selected to achieve ½crystal physical resolution and interpolation is performed of detectedreflected ultrasound pulses to achieve ¼ crystal digital resolution. 17.The ultrasound probe of claim 15, wherein the selection includesperforming at least one of varying the number of piezoelectric crystalsdriven and shifting a position of piezoelectric crystals activated. 18.The ultrasound probe of claim 17, wherein the selection includes varyinga number of active piezoelectric crystals over the firing sequence. 19.The ultrasound probe of claim 15, wherein the selection includes varyinga spatial distribution of active piezoelectric crystals over the firingsequence.
 20. The ultrasound probe of claim 19, wherein the scanconversion electronics applies a time varying focus coefficients for thedetected ultrasound pulses to improve spatial focus.
 21. The ultrasoundprobe of claim 15, wherein the selection comprising sequentially varyingactive piezoelectric crystals over the firing sequence to move thetransmitted beam focal point by ½ crystal at a time so that the centerof distribution of the pulse is shifted ½ crystal at a time in eachfiring sequence.
 22. The ultrasound probe of claim 15, wherein the scanconversion electronics performs scan line conversion of data receivedfrom reflected ultrasound pulses, including performing, in a scan linedomain, interpolation to increase a number of samples in scan lineconversion.
 23. The ultrasound probe of claim 22, wherein the scanconversion electronics performs interpolation to increase a number ofsamples in scan line conversion.
 24. The ultrasound probe of claim 23,wherein the scan conversion electronics performs interpolation in radialzones to increase a number of samples in a distal zone region withrespect to the transducers.
 25. The ultrasound probe of claim 15,further comprising a calibration module to perform auto-calibration ofthe ultrasound probe.
 26. An ultrasound probe, comprising, a probehousing; an array of piezoelectric transducers disposed in a transducerhead detachably attached to the probe housing, wherein eachpiezoelectric transducer in the array includes a piezoelectric crystal;beamforming means in the probe housing to shape a gain and a delay ofvoltage pulses coupled to the array of the piezoelectric transducerswith a firing sequence in which a selection of active piezoelectriccrystals is varied over time in a sequence selected to shift a spatialcenter of a transmitted ultrasound pulse over the firing sequence toachieve at least ½ piezoelectric transducer crystal physical resolution;and scan conversion means in the probe housing including interpolationmeans to increase a number of samples in scan conversion.
 27. A methodof providing super-resolution in an ultrasound imaging system having ahandheld probe including piezoelectric transducer having an array ofpiezoelectric crystals, comprising: generating ultrasound pulses,including generating voltage pulses within the handheld probe in afiring sequence to drive the array of piezoelectric transducer crystalsis selected such that a gain and a delay of each voltage pulse coupledto the array of piezoelectric transducer crystals splits the resolutionto sub-pixel accuracy in which a selection of active piezoelectriccrystals is varied over time in a sequence selected to shift a spatialcenter of a transmitted ultrasound pulse over the firing sequence toachieve at least ½ piezoelectric transducer crystal physical resolution;detecting reflected ultrasound pulses; performing scan conversion withinthe handheld probe for detected reflected ultrasound pulses, includingperforming, in a scan line domain, interpolation of samples for scanline processing used to reconstruct the ultrasound image; and outputtingultrasound images.
 28. The method of claim 27, wherein the interpolationcomprises at least one of wavelet interpolation and splineinterpolation.
 29. The method of claim 27, wherein the interpolationcomprises an orthonormal expansion sampling method.
 30. The method ofclaim 29, wherein the orthonormal expansion is selected from the groupconsisting of Legendre sampling, Tchebyshev sampling, and Sinc sampling.